2A Modified for Drives, Pads and Turnarounds in Pennsylvania

West of the Alleghenies the ground is flat-lying shale, siltstone and claystone weathered into silty, clay-rich soil, sitting on hills cut by steep valleys. A drive built on that does not give up in August. It gives up in the third week of March, when the frost comes out of the ground, the clay underneath softens, and the surface is still being asked to carry a loaded truck. 2A modified is the layer that stands between those two things: a compacted, load-spreading course that keeps wheel weight off soft subgrade and keeps water moving instead of sitting.

Reading a pile of 2A modified: one inch minus, fines left in

The load arrives as a graded mix of crushed stone, gravel and binding fines, sized one inch minus, which means the largest pieces run about an inch and everything below that is in the blend right down to dust. Grab a handful dry and it runs through your fingers with a gray film left behind. Grab the same handful damp and it balls up. That is the whole point of the material. The angular stone carries the load, the fines fill the gaps between the stone, and water plus compaction locks the two together into a dense layer.

Clean stone does the opposite. Tip 2B or #57 out of a bucket and it rattles, because every piece is roughly the same size and the voids between them stay open. Voids are what you want in a drain. They are not what you want under a truck.

Locally this stone is specified and sold by the older Pennsylvania size names alongside the AASHTO numbers. PennDOT Publication 408, Section 703 names a dense-graded No. 2A subbase, and yards use 2A modified as the trade name for a 2A-type base with extra fines added. That is how the material is described and ordered here, not a certification carried by anything on this site.

Why hillside lanes in Southwestern Pennsylvania get this stone first

Most orders that leave as 2A Modified are going under something. A farm lane that ruts every mud season. A parking pad beside a shop. A turnaround at the end of a long drive. A shed floor, a patio base, an equipment pad beside a barn. In each case the stone is not the thing you look at. It is the thing that stops the thing you look at from moving.

On plateau hills the second job is drainage, and it is not optional. Water soaking into clay layers on a slope is the main landslide trigger in this part of the state, so where roof and drive runoff ends up matters as much as how thick the base went down. Crown the surface, give the water a ditch to leave by, and keep downspouts from discharging onto the edge of a pad.

Jobs on the same lot where modified stone is the wrong answer

The fines that make this material compact are exactly what disqualifies it elsewhere.

• A French drain, a curtain drain or the backfill behind a wall wants open voids. Order 2B Stone or #57 Stone instead, and keep the modified out of the trench.
• A bed that has to look finished takes River Rock or mulch, not gray dust.
• A garden path takes a finer, walkable stone. Walkway Gravel is the listing for that.
• Raising a low spot before any of this starts is a fill job, and Fill Dirt is the ground to make grade up with rather than base stone.

Depth on clay hillside ground, and what frost has to do with it

Frost reaches about 36 inches through most of Pennsylvania, from roughly 30 inches in parts of the southeast to 42 inches in the north and the mountains, set by each municipality. Nobody builds a gravel drive to that depth. Footings go below frost; a drive cannot. What a drive does instead is drain, so that the water which freezes and thaws over and over each winter is not sitting inside the layer while it does it.

The depth guide on the calculator puts landscaping work around 2 inches and driveway work around 4. Treat 4 inches as the starting point for a passenger-car drive over ground that is already firm and draining. A pad that will see a loaded truck, a tractor or a trailer wants more than that, built up in lifts rather than one deep dump, with each lift wetted and compacted before the next goes on. Soft, wet clay pushes the figure up again, and wants the problem underneath fixed first.

Two rules hold everywhere on this ground. Base stone does not go down on frozen or muddy ground. And a layer that was never compacted is not a base, it is a pile you drive on.

Sizing a 20 by 24 turnaround, a hot tub pad and a dog run

These are hypothetical examples, worked so the method copies onto your own measurements. The arithmetic is area in square feet, times depth converted to feet, divided by 27.

The road base calculator adds 10 percent for compaction and spillage by default, which moves the turnaround from 8.9 to about 9.8 cubic yards. At the weight published on this site's calculator pages for crusher run and road base, about 3,000 lb per cubic yard or roughly 1.50 tons per yard, that 9.8 yards is approximately 15 tons (9.8 x 1.50 = 14.7). Stone is bought by weight rather than by volume, so run the same figures through the road base calculator to turn cubic yards into tons before you set a quantity.

The layers either side of a compacted base

Underneath, anything soft or organic comes out before stone goes in, and low ground gets made up with fill so the base is not being asked to do two jobs at once. On very soft subgrade, a separation fabric under the stone stops the fines from disappearing into the mud.

On top, the choice depends on what the surface has to do. A working lane or a parking area can be topped with Driveway Gravel and left at that. A patio or a paver apron takes a thin bedding course of Screenings over the compacted base, screeded flat, with the pavers set into it. Nothing laid on top will hide a base that went down thin.

Setting a quantity and a date for a modified stone load

Work out your cubic yards, convert them, then pick the quantity and the delivery date you want on this page. Delivery is arranged at checkout from your ZIP code. Before the day, clear a dump spot the truck can reach and back out of without crossing soft ground, and mark it so the driver is not guessing. If the lane is narrow or the grade is steep, say so through Contact and Pennsylvania Rock Supply can work out how the load is best split.